Short answer: sin.
The world is not the place God intended, because people are in rebellion against God. Some sin hurts others; some sin hurts yourself. Either way, you can't blame God for the harm that comes when people do exactly what he said not to do. If you are lazy and a drunkard and so can't hold down a job, perhaps the fault is with yourself and not with God. If a sadistic murderer kills an innocent person, perhaps the fault is with the killer and not with God.
There are some verses that are often quoted to say that the Bible promises that once you become a Christian your life will be problem-free. There's Matthew 6 that you quote in your question. There's Philippians 4:13, "I can do all things through Christ[a] who strengthens me." Romans 8:28, "And we know that all things work together for good to those who love God." Etc.
But common sense would tell us that the Bible writers could not possibly have meant that the Christian-life is trouble-free. They all were well aware that this is not so. The Philippians 4:13 verse I quoted above comes right after 4:12, "I know how to be abased, and I know how to abound. Everywhere and in all things I have learned both to be full and to be hungry, both to abound and to suffer need." Paul was well aware that all Christians are not healthy and wealthy, from personal experience if nothing else. The same Paul who wrote the Philippians and Romans verses I quoted also wrote 1 Cor 11:23-28, in which he recounts how he has been shipwrecked, beaten, imprisoned, etc etc. Likewise, the same Jesus who said the words in Matthew 6 also said, just a page or two back in Matthew 5:10-11, "Blessed are those who are persecuted for righteousness’ sake, For theirs is the kingdom of heaven. Blessed are you when they revile and persecute you, and say all kinds of evil against you falsely for My sake." He was well aware that his followers would be slandered and tortured and killed for their beliefs.
I've heard lots of people say that they don't believe the Bible because there is so much evil and suffering in the world. But that makes no sense at all. Nowhere does the Bible say that the world is a place of perfect happiness and prosperity. Quite the contrary, the Bible repeatedly says that the world is full of evil and sin and suffering. We look at the world around us and we see that it is full of evil and suffering. Thus, the Bible has been proven wrong! Um, what? These people have made up their own religion, a religion where God prevents all evil and suffering in the world, a religion completely different from Biblical Christianity. Then they look at the world around them and see it does not match this religion. And so they declare Biblical Christianity to be false.
By the way, I think the experience of history is that to the extent that people live according to God's laws, they are free and healthy and prosperous. While of course there is poverty and crime and suffering all over the world, there is a whole lot less of it in places where Judaism and Christianity have historically been strong, like Europe and North America and Israel, than in places where Judaism and Christianity are weak, like Asia and Africa and the Middle East outside of Israel.